Hydrogen energy systems have attracted attention from the viewpoints of reducing carbon dioxide emissions and promoting clean energy. When hydrogen is used as an energy carrier, electricity and heat can be produced from hydrogen in a fuel cell, while heat and power can be produced by direct combustion of hydrogen. In these cases, safe and innocuous water is obtained as a final product, a clean energy cycle can be created. In most cases, hydrogen as an energy carrier is produced from petroleum or natural gas by catalytic cracking, although it occurs naturally as well. It also is possible to produce hydrogen and oxygen by water electrolysis, but this cannot be a fundamental solution because the electrolysis requires electrical energy. It also is possible to use a system in which a solar cell converts light energy into electricity so as to perform electrolysis with this electric power. However, in terms of the production cost, energy consumption, and electricity storing capacity of the solar cell, the hydrogen production method using such a system is not necessarily an effective one.
In contrast, hydrogen production using a photocatalyst is a system for producing hydrogen directly from water and sunlight, and can convert the solar energy effectively into hydrogen energy.
Several proposals have been made about the configuration of a hydrogen production device using a photocatalyst. For example, Patent Literature 1 proposes a hydrogen production device including a photocatalyst layer disposed on the bottom of a reaction vessel and a water layer flowing on the photocatalyst layer. In this device, water in the water layer is decomposed by irradiating the photocatalyst layer with light so as to obtain oxygen gas and hydrogen gas. Patent Literature 2 proposes a hydrogen production device in which an oxygen producing electrode side region and a hydrogen producing electrode side region in a cell are separated from each other by a conductive substrate and an ion exchange membrane. Patent Literature 3 proposes a hydrogen production device having a structure in which a positive electrode, including a substrate with small through-holes, and a platinum thin film and a photosemiconductor thin film formed thereon, and a negative electrode having a platinum layer are bonded to each other with an ion exchange membrane interposed therebetween. In this hydrogen production device, the positive electrode and the negative electrode are electrically connected through an external circuit.